1. Field of the Invention
The present invention relates to a crack resistant layer with good binder fracture energy properties and a method of selecting same. More particularly, the present invention relates to a bituminous binder with a critical amount of conjugated diene, which allows for enhanced binder fracture energy properties in a crack resistant layer.
2. Description of the Related Art
When pavements deteriorate, they may be overlaid with hot mix asphalt (HMA) to repair them. When designing an overlay, the rate of crack propagation through the overlay, the rate of deterioration of the reflective crack, and the amount of water that can infiltrate through the cracks must be considered. One disadvantage with such HMA overlays is that cracks in the old pavement reflect through the new overlay. To relieve this reflective cracking, thicker overlays may be placed. Another disadvantage with these overlays is that they typically have a low strain tolerance and a low resistance to reflective cracking. To improve traditional HMA overlays, asphalt binders that display the ability to undergo creep or stress relaxation at low temperatures may be used. Such bituminous binders minimize the potential for thermal and reflective cracking. However, the disadvantage with such bituminous binders is that they are highly ductile and have a low shear modulus at high temperatures, and thus roads created with them tend to rut. Asphalts with high shear modulus that resist rutting at high temperatures may also be used. However, such binders tend to be brittle at low temperatures, and thus roads created with them tend to crack. Typical asphalt bituminous binders formulated for pavement applications usually display either high shear modulus at high temperatures or high ductilities at low temperatures but not both.
A typical highway HMA surface mixture has about 3% to 5% air voids and a fatigue life of only about 500 cycles when tested at 15° C. with a strain amplitude of 2,000 microstrains and frequency of 10 Hz using a 4-point bending beam apparatus. The best surface mixture with about 3% to 5% air voids has a fatigue life of only about 2,000 to 5,000 cycles when tested at 15° C. with a strain amplitude of 2,000 microstrains and frequency of 10 Hz using a 4-point bending beam apparatus. Other mixtures with air voids greater than 5% to 7% may have a fatigue life of only about 500 to 1,500 cycles when tested at 15° C. with a strain amplitude of 2,000 microstrains and frequency of 10 Hz using a 4-point bending beam apparatus.
Blankenship et al., U.S. Pat. No. 6,830,408, which is incorporated herein by reference, attempts to solve the foregoing problems through the use of an interlayer that is placed on the cracked road underneath the overlay. The interlayer includes a mixture of aggregate and bituminous binder, preferably polymer modified asphalt, and is used to delay or stop the occurrence of cracking, control crack severity, reduce overlay thickness, and enhance waterproofing capabilities. The interlayer is highly strain tolerant and substantially impermeable.
The bituminous binder used in the interlayer of the '408 patent includes bitumen, one or more polymers, and, optionally, a cross-linking agent to effect vulcanization of the polymer in the bitumen. Limitations on the characteristics of the bituminous binder and interlayer are set forth in the '408 patent. In particular, the '408 patent specifies that the percentage of air voids in the interlayer must be between 2.0% and 4.0%. This produces a flexural beam fatigue performance of at least 100,000 cycles to failure.
The problem with such interlayers is that, in order to get such a fatigue life and retard the progression of reflective cracks in the pavement, these interlayers sacrifice a degree of their load bearing capacity, as measured in the Hveem stabilometer, and typically have Hveem stabilities of about 18-21. In order to compensate for their low stability, these interlayers are placed below the top layers of a pavement structure so that they are not exposed to direct traffic loads. Thicker top layers help to improve the total structural stability but are costly. Still further, the top layers of the pavement structure cannot completely compensate for the low load bearing capacity of the interlayer.
Blankenship et al., U.S. application Ser. No. 10/631,149, which is incorporated herein by reference, attempts to solve this problem through the use of a highly strain tolerant, substantially moisture impermeable, hot mix reflective crack relief interlayer. The interlayer includes a polymer modified bituminous binder mixed with a dense fine aggregate mixture that is made primarily from manufactured sand. This results in increased stability and improved load bearing capacity. Limitations on the characteristics of the bituminous binder and interlayer are set forth in the '149 application. In particular, the '149 application specifies that the percentage of air voids in the interlayer must be between 1.0% and 5.0%, preferably 2.0% to 4.0%, and most preferably about 3.0%. This produces a flexural beam fatigue performance of at least 50,000 cycles to failure, preferably 80,000 and most preferably 100,000.
The problem with this interlayer is that it is impermeable. When such an interlayer is placed on Portland Cement Concrete (PCC) or another paved surface, the interlayer has the potential to trap vapor underneath it. As changes occur in climatic and environmental conditions, this causes the PCC to release moisture or vent. The interlayer then rises, creating a blister. This causes overlays on top of this interlayer also to rise and blister.
Blankenship et al., U.S. Pat. No. 7,479,185, which is incorporated herein by reference, attempts to solve this problem through the use of a layer that remains substantially moisture impervious and retains its ability to retard the formation of reflective cracks while having increased vapor permeability. This layer may be an interlayer, but also may be a base layer or an overlay.
Limitations on the characteristics of the bituminous binder and layer are set forth in the '185 Patent. In particular, the '185 Patent specifies that the percentage of air voids in the layer must be at least 3.0%, preferably at least 4.0%, more preferably at least 4.5%, even more preferably at least 5.0%, and most preferably at least 7.0%. This produces a flexural beam fatigue performance of at least 5,000 cycles to failure, preferably at least 35,000 cycles to failure, and most preferably at least 100,000 cycles to failure. The '185 Patent notes that there is typically an inverse relationship between the air voids in a bituminous mixture and fatigue resistance of that mixture. However, the bituminous mixture of the '185 Patent may be made by creating a very large amount of air voids in an aggregate structure and then filling a large portion of those voids with bitumen. The total amount of air voids is critical. Too many air voids will limit fatigue resistance and too few air voids will compromise permeability.
The problem with the '185 layer is the narrow operating window. The perfect aggregate structure is required to produce the skeletal structure that meets the requirements of fatigue resistance, strength, and permeability. Local aggregates may not be suitable requiring more costly aggregate sources to be used. Tight tolerances at the hot mix plants creates off-specification product that impacts costs. Additionally, a very high asphalt content is required, which increases costs dramatically.
The current art uses mixture volumetric properties and film thickness to achieve acceptable beam fatigue properties. The '408 patent to Blankenship requires air voids in a tight and low range, extremely high binder film thicknesses, and extremely low DP's (dust to effective binder ratio). The '149 application greatly limits aggregate properties to effect acceptable beam fatigue properties. The '185 Patent allows for higher air void content but also requires a higher binder film thickness. Hence, the current art is void of any binder property that affects beam fatigue properties.
In each of the foregoing, polymer is used in the bituminous binder. Methods of preparing polymer modified bitumen is described in Maldonado et al., U.S. Pat. No. 4,242,246, and Maldonado et al., U.S. Pat. No. 4,330,449, both of which are incorporated herein by reference.
Notwithstanding the foregoing, there remains a need for a crack resistant layer with low air voids and good binder fracture energy properties that does not suffer from the drawbacks of the layers of the Blankenship patents and application. Accordingly, it would be desirable to provide a bituminous binder for a crack resistant layer with greater than 1% air voids and binder fracture energy greater than 40 J/m2, that is stable, that does not require special aggregate structure or excessive asphalt content, and that may be used as a base layer, interlayer, or overlay.